1. Technical Field
The present disclosure relates to building insulation and more specifically to creating a continuous air impermeable barrier across a body of fibrous insulation to better reduce the flow of conditioned air out of a building envelope.
2. Introduction
Typical A-frame building design relies on an insulated attic to help retain heated or cooled air. Without attic insulation, temperature-regulated air would easily escape through the structure due to conduction. While insulation can help slow the loss of conditioned air, the conditioned air can also escape a structure if there are gaps and penetrations between conditioned and unconditioned spaces where air can easily transit. Traditional fibrous insulation (e.g. fiberglass, mineral wool or cellulose) is typically not air impermeable, and any gaps or openings in a building envelope that are not covered with an air barrier can allow air to easily flow through fibrous insulation, thereby stripping it of most, if not all, of its ability to retain conditioned air. Unconditioned attic spaces can be particularly susceptible to the effects of airflow caused either by wind or by convection. In the case of wind, airflow through a structure is affected by the force of wind blowing against a structure and subsequently being pushed or pulled through gaps in the building envelope. For example, unconditioned attics can accelerate the loss of conditioned air from within the conditioned space as they are often heavily ventilated and the ventilation can produce a wicking effect on the fibrous insulation located on the attic floor. In addition to wind, convective forces can produce internal pressure differentials that can push or pull air through a building envelope as well. In the building sciences world this is often referred to as the “stack effect”. In cooler temperatures, less dense heated air can travel up through ceiling penetrations (light fixtures, gaps in vents, shafts, piping, etc.) and ultimately out through an unconditioned attic space. The reverse can also occur during warmer periods when denser air conditioned air causes warmer outside air to be drawn into a structure from unconditioned attic spaces and pushed out through gaps in the building envelope below the attic.
To combat the stack effect and to enhance the performance of fibrous insulation, insulators often utilize a combination of air and vapor barrier materials and careful sealing of gaps and penetrations to impede airflow between the internal conditioned living space and the unconditioned portion of an attic. This air sealing labor can easily exceed the cost of the insulation material and its installation. Still, if the air sealing is not performed utility bills for heating and cooling can be 30-50% higher in some situations.
Ventilation in an unconditioned attic space is also critical to preventing condensation and other moisture accumulation from causing wood rot and mold problems on building materials (especially the underside of roof sheathing and along roof trusses). As a result, soffit vents, ridge vents and gable vents are usually employed to help enhance airflow through an unconditioned attic space. Often ventilation baffles are utilized to provide conduits for air entering an attic from soffit vents to travel up the interior side of a roof and escape near the apex of the house through ridge vents, thereby helping to regulate temperatures within the attic and to help manage moisture accumulation on the underside of roof sheathing. Ventilation baffles can also prevent insulation from being in direct contact with the roof sheathing, which can impede airflow at soffits vents and produce a moisture problem that can eventually lead to mold and/or wood rot. While ventilation baffles are important they often are not sufficient to significantly reduce or eliminate “wind washing” which can occur when wind driven air is forced into the attic through attic vents and this air then pulls conditioned air out of exposed fibrous insulation. In some cases as much as 25% or more of conditioned air lost through an attic can be attributed to wind washing.